Double hit if you're at or near your ATs damage cap. +damage if not regardless of the AT IMO.
A blaster at damage cap is still at damage cap. The only way to do more damage is by ignoring cap with double hit, procs etc. +damage goes into cap #s while double hit does not. A kin can put most ATs at or near cap if my memory is correct. If you run solo (mostly) and aren't at cap then +damage most likely.
Maximum Damage:
Brute 775%
Blaster, Corruptor,
Scrapper, Stalker 210% at level 1 – 500% at level 21+
all others 200% at level 1 – 400% at level 21+
Source - limits
Notes:
Attacks do 100% damage to start with (i.e. base, or 1x normal), not 0%. The amount of Damage bonus needed to reach the cap is 100 less than the cap itself.
These maximums apply only to effects that increase the amount of damage an attack delivers. Reductions to an enemy's Resistances can cause that enemy to suffer damage beyond these maximums. So can effects that cause attacks to deliver damage more than once, such as Containment, Scourge, and Critical Hit.
A Blaster for instance has an actual cap of 400% not 500% since attacks "do 100% damage to start with".
I use combat attributes to monitor my +damage percentage.